The present invention relates to a method for producing a structure that includes a thin layer of semiconductor material on a substrate. The method includes the steps of:
carrying out implantation of species under a face of a donor substrate from which the thin layer must be made, so as to create an embrittlement zone in the thickness of the donor substrate,
placing the face of the donor substrate into close contact with a support substrate, after it has undergone implantation,
detaching the donor substrate at the level of the embrittlement zone, so as to transfer a part of the donor substrate onto the support substrate and to form the thin layer on the latter.
The invention relates more precisely to the above-mentioned implantation step.
SMARTCUT® type processes, for which more ample details can be found in the document ‘Silicon-On-Insulator Technology: Materials to VLSI, 2nd Edition’, by Jean-Pierre Colinge from Kluwer Academic Publishers, pp. 50 and 51, are an example of methods of the type mentioned hereinabove and correspond to a preferred embodiment of the invention
Such processes advantageously produce structures comprising a thin layer of semiconductor material, such as SeOI (Semiconductor On Insulator) structures and the like.
The resulting structures from such processes are used for applications in the field of microelectronics, optics and/or optronics.
Implantation of species is understood to mean any technique (such as bombardment, diffusion and the like) suitable for introducing atomic or ionic species onto the material of the implanted donor substrate, with a maximum concentration of the implanted species situated at a preset depth from the substrate relative to the surface of the implanted substrate.
The implantation step can be performed by co-implanting at least two different species. A common advantage of the co-implantation technique is the reduction by a factor of 3 approximately of the implanted dose relative to the implantation of a single type of species.
For instance, it is established in the article by Aditya Agarwal, T. E. Haynes, V. C. Venezia, O. W. Holland, and D. J. Eaglesham, “Efficient production of silicon-on-insulator films by co-implantation of He+ with H+”, Applied Physics Letters, vol. 72 (1998), pp. 1086-1088, that the co-implantation of hydrogen H and helium He enables thin layer detachment at a much lower total implantation dose than that required when either hydrogen or helium alone is implanted. This reduction translates by reduction in the implantation time, and finally in costs associated with production of structures comprising a thin layer on a support substrate, in particular by means of a SMARTCUT® type transfer process. As it is mentioned in this article, the essential role of hydrogen H is to interact chemically with the implantation damage and to create H-stabilized platelet-like defects, or microvoids, in the implanted donor substrate. In the other hand, helium He plays a predominantly physical role and acts as a source of internal pressure in the implanted donor substrate for providing stress in said defects. Thus, He drives growth and eventual intersection of the microvoids so that two continuous separable surfaces may thereafter form on each side of the embrittlement zone.
The face of the donor substrate under which implantation is carried out is thereafter placed into close contact with the face of the support substrate, in order for bonds to create in between said faces at their interface called bonding interface. However, if particles or organic substances are present on the surfaces to be bonded, they may prevent the bonding to efficiently take place at certain sites at the bonding interface and voids may thus appear between the bonded substrates. Furthermore, the implanted species can thus easily diffuse into these voids, thus forming blisters at the bonding interface. This is in particular the case when the structure obtained after the donor and support substrates have been placed into close contact is subjected to a thermal treatment intended in particular to strengthen the bonding or to detach the donor substrate at the level of the embrittlement zone. Moreover the presence of voids can lead to the establishment of imperfectly bonded areas where the bonding strength may not be sufficient for allowing the detachment of the donor substrate at the level of the embrittlement zone so that some areas (called “non transferred zones”) of the thin layer may not be transferred onto the support substrate.
In addition, it has to be noted that the implantation step degrades the quality of the surface of the face of the donor substrate under which implantation is carried out and which is to be placed into close contact with the donor substrate, thus increasing the risk that blisters are formed at the bonding interface and that zones are not transferred onto the support substrate. Blisters are not desired since they decrease the effective usable wafer surface area and hence decrease the production yield. Wafers presenting blisters are even rejected from the production line. Consider a donor substrate made of silicon Si (and which may optionally comprise a superficial SiO.sub.2 layer), wherein the substrate has undergone co-implantation of He and H atoms. It is generally known that the risk of blister formation increases if He is implanted close to the bonding interface. Thus, a number of methods are usually carried out in order in an attempt to avoid blister formation.
A first method relates to implanting He atoms deeper than H atoms within the donor substrate (starting from the donor substrate's face under which implantation is carried), that is by providing an adequate He implantation energy which can be calibrated thanks to SIMS analysis. Another method relates to increasing the H dose to be implanted, typically by a couple of 1015 H atoms/cm2. Of course both these methods can be implemented jointly. The effect of these methods is illustrated by Table 1 below which represents the number of blisters detected as a co-implantation of He and H atoms has been performed under the following co-implantation conditions:                a dose of He atoms of 12×1015/cm2;        a He implantation energy as indicated along the Y axis, that is 34, 40 and 46 keV;        a dose of H atoms implanted as indicated along the X axis, that is 9, 12 and 15×1015/cm2;        a H implantation energy of 27 keV.        
TABLE 1
In the lowest row, corresponding to the smallest He implantation energy and thus to the shallowest He implantation depth, blister formation is observed. However as the He implantation energy increases (He being thus implanted deeper), less blister formation is observed. In other words, the deeper He is implanted, a reduced blister formation is observed. In the left-hand side column of the Table, corresponding to the lowest H dose, blister formation is observed. However, as the H dose increases (see center and right hand side columns), blister formation decreases. In other words, the higher the H dose is, a reduced blister formation is observed. In both methods, the H implanted region is regarded as acting as a gettering region or barrier making it possible to block the diffusion of He towards the bonding interface. As mentioned above, the donor substrate is detached at the level of the embrittlement zone created in its thickness by the implantation step so as to transfer a part of the donor substrate onto the support substrate and to form the thin layer on the latter.
The specifications of the surface state of the structures obtained by transfer processes such as the SMARTCUT® process are generally very strict. Indeed, the roughness of the thin layer is a parameter which to a certain extent conditions the quality of the components which will be created on the structure. There is thus a need for limiting as much as possible the surface roughness of the thin layer, and thus to implement the implantation step under conditions making it possible to limit the roughness. Table 2 presented below shows the surface roughness measured after the detachment step has been performed and the resulting structure has been subjected to a RTA (Rapid Thermal Annealing) adapted for gumming out certain roughnesses by surface reconstruction. The co-implantation conditions are the same as those exposed in relation with Table 1. The surface roughness has more precisely been measured on a surface of 10×10 μm2 swept by the point of an atomic force microscope AFM and is expressed by an average quadratic value known as RMS (Root Mean Square).
TABLE 2
It is apparent from this table that the two best conditions for limiting the roughness are those underlined on the upper part of the left hand side column. However these conditions result, as shown in Table 1, in the formation of blisters. On the other hand, conditions that do not cause blisters to form are conditions which also do not limit roughness. Hence, as the comparison of Table 1 and Table 2 makes it clear, certain implantation conditions that result in the best roughness may lead to undesirable blister formation and, reciprocally, conditions that avoid blister formation may result in poor surface roughness. It thus appears that surface roughness and blister formation cannot be controlled separately. Hence a compromise has to be made between the best conditions (e.g., implantation energies and doses) for avoiding blister formation and the best conditions for limiting the resulting surface roughness. It is mentioned here that such a compromise may have to be conducted for controlling not only roughness but also other parameters such as the thickness homogeneity of the transferred thin layer, the thickness of the damage zone, the splitting temperature, etc.
There is thus a need for a method for producing a high quality structure comprising a thin layer of semiconductor material on a substrate for which the co-implantation conditions are optimally controlled, in particular in order both to avoid blisters formation and to limit the resulting surface roughness.